ClusterXL - Adding Interface to External Switches

Hi. This is possibly going to sound like a stupid question so apologies in advance (newbie). We have ClusterXL setup for ISP redundancy and I need to configure some additional public IPs from our primary ISP. When connecting a link from each of our firewalls to the external switches, do I need to/how can I access the external switches to configure the additional link? I'm assuming it's not just a case of connecting up the ethernet between firewalls and switch?